Method for postdoping a semiconductor wafer

ABSTRACT

A method for treating a semiconductor wafer having a basic doping is disclosed. The method includes determining a doping concentration of the basic doping, and adapting the basic doping of the semiconductor wafer by postdoping. The postdoping includes at least one of the following methods: a proton implantation and a subsequent thermal process for producing hydrogen induced donors, and a neutron irradiation. In this case, at least one of the following parameters is dependent on the determined doping concentration of the basic doping: an implantation dose of the proton implantation, a temperature of the thermal process, and an irradiation dose of the neutron irradiation.

REFERENCE TO RELATED APPLICATION

This application claims priority to German application number 10 2013216 195.6 filed on Aug. 14, 2013.

FIELD

Example embodiments of the present disclosure relate to a method fortreating a semiconductor wafer, in particular for doping a semiconductorwafer.

BACKGROUND

Semiconductor components having a high dielectric strength, i.e.semiconductor components having dielectric strengths of from a few tensof volts (V) to a few kilovolts (kV), are widely used in many fields,such as, for example, industrial electronics, automotive electronics orconsumer electronics. Semiconductor components having a high dielectricstrength which are able to carry high currents, such as, for example,currents of a few amperes or more, in the on state are also designatedas power components. Semiconductor components having a high dielectricstrength include, for example, MOSFETs (Metal Oxide SemiconductorField-Effect Transistors), IGBTs (Insulated Gate Bipolar Transistors),bipolar transistors, bipolar diodes, thyristors or Schottky diodes.

These components have a relatively lightly doped semiconductor region,usually designated as the drift region (in the case of MOSFETs) or asthe base region (in the case of diodes or thyristors). This driftregion/base region forms a pn junction or a Schottky junction withanother component zone, such as, for example, a body region in the caseof a MOSFET or an IGBT, and is able to take up a space charge zone inthe case of a reverse-biased pn junction/Schottky junction. The reversevoltage strength, that is to say the voltage which can maximally beapplied in the reverse direction before a critical field strength isattained and an avalanche breakdown commences, is dependent, inter alia,on a doping concentration of the drift region/base region and thedimension thereof in a direction perpendicular to the pnjunction/Schottky junction.

In a semiconductor component having a high dielectric strength, thedrift region/base region occupies a significant part of the volume of asemiconductor body in which the semiconductor component is implemented.This applies in particular to a vertical semiconductor component, thatis to say a component in which the drift region/base region is arrangedbetween further component zones (for example the body zone and the drainzone in the case of a MOSFET) situated in the region of opposite sidesof the semiconductor body. For the production of such a semiconductorcomponent, therefore, it is desirable to have available a semiconductorsubstrate having a basic doping that already corresponds to the desireddoping of the drift region/base region. Further doped component regionscan then be produced by conventional doping methods in the semiconductorsubstrate, those regions in which the basic doping is maintained formingthe drift region/base region.

On account of the abovementioned dependence of the dielectric strengthof the component on the doping of the base region/drift region,providing a semiconductor substrate having an exactly defined basicdoping is of great importance.

In order to reduce costs when producing semiconductor components,usually a multiplicity of identical components are producedsimultaneously on the basis of a semiconductor wafer. Said semiconductorwafer forms a semiconductor substrate for a multiplicity of componentsand is divided into individual semiconductor chips (referred to as dies)after processing.

Such semiconductor wafers for the production of semiconductor componentsare obtained by sawing a cylindrical (rod-shaped) single crystal. Knownmethods for producing such a single crystal include the Czochralski (CZ)method, the magnetic Czochralski (MCZ) method or the float-zone (FZ)method. The single crystal can be doped during the production method. Inthis case, by means of the FZ method, a single crystal having a veryhomogeneous and defined doping can be produced, which can be subdividedinto semiconductor wafers suitable as substrates for the production ofhigh-voltage components. However, heretofore, single crystals producedaccording to the FZ method have been available only with a diameter of8″ (inches). In order to increase efficiency, however, it would bedesirable to process semiconductor wafers having a higher diameter, suchas 12″, for example, in order to be able to produce a higher number ofcomponents simultaneously.

Heretofore, however, single crystals having such higher diameters havenot been able to be produced according to the FZ method. Although suchsingle crystals can be produced by the MCZ method, the single crystal isalready doped during the production method, thus resulting in a veryinhomogeneous doping which decreases greatly from a first longitudinalend to a second longitudinal end of the semiconductor rod. Furthermore,the maximum doping present at the first longitudinal end can alsofluctuate from single crystal to single crystal under identicalproduction conditions.

SUMMARY

In one embodiment of the present disclosure, a semiconductor waferhaving a large diameter, such as, for example, 12″ or more is provided,for the production of semiconductor components, in particular componentshaving a high dielectric strength.

In one embodiment, the disclosure relates to a method for treating asemiconductor wafer having a basic doping. The method comprisesdetermining a doping concentration of the basic doping, and adapting thebasic doping of the semiconductor wafer by postdoping. Said postdopingcomprises at least one of the following methods: a proton implantationand a subsequent thermal process for producing hydrogen induced donors;and a neutron irradiation. During the postdoping, at least one of thefollowing parameters is dependent on the determined doping concentrationof the basic doping: an implantation dose of the proton implantation, atemperature of the thermal process, and an irradiation dose of theneutron irradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments are explained in greater detail below with referenceto drawings. These drawings serve to explain the principle, and so thedrawings illustrate only those features which are necessary forunderstanding the principle. The drawings are not true to scale. In thedrawings, unless indicated otherwise, identical reference signsdesignate identical features having the same meaning.

FIG. 1 schematically shows a cylindrical (rod-shaped) single crystal.

FIG. 2 schematically illustrates the resistivity and the dopingconcentration of a single crystal produced according to the MCZ methodover the length of the single crystal.

FIG. 3 illustrates one example embodiment of a method for measuring theresistivity of a semiconductor wafer obtained from a cylindrical singlecrystal.

FIGS. 4A and 4B illustrate one example embodiment of a method formeasuring the resistivity of a cylindrical single crystal.

FIG. 5 illustrates one example embodiment of a method for postdoping asemiconductor wafer.

FIG. 6 schematically shows a plan view of a semiconductor wafer andillustrates a grid for later division of the semiconductor wafer.

FIG. 7 shows a vertical cross-sectional view of a part of asemiconductor wafer in which component zones of a MOS transistor havealready been produced, during the postdoping.

FIGS. 8A and 8B show a vertical cross-sectional view of a part of thesemiconductor wafer in which component zones of a MOS transistor havealready been produced, during a postdoping in accordance with a furtherexample.

FIG. 9 shows a vertical cross-sectional view of a part of asemiconductor wafer in which component zones of a thyristor have alreadybeen produced, during the postdoping.

FIG. 10 shows a vertical cross-sectional view of a part of asemiconductor wafer in which component zones of a bipolar transistorhave already been produced, during the postdoping.

FIG. 11 shows a vertical cross-sectional view of a part of asemiconductor wafer in which component zones of a Schottky diode havealready been produced, during the postdoping.

FIG. 12 illustrates one example embodiment of a further method forpostdoping a semiconductor wafer.

FIG. 13 illustrates one example embodiment of yet another method forpostdoping a semiconductor wafer.

DETAILED DESCRIPTION

FIG. 1 schematically shows a cylindrical monocrystalline semiconductorbody 1, which is also designated as a semiconductor rod hereinafter.This semiconductor rod is, for example, a semiconductor rod producedaccording to the MCZ (magnetic Czochralski) method and has a diameter dand a length l. The diameter d is for example more than 8 inches, suchas, for example, 12 inches (approximately 30.48 cm) or more. Such asemiconductor rod 1 can be doped as early as during the productionmethod, i.e. during the pulling of the semiconductor rod from a melt.However, such semiconductor rods that are also doped during theproduction method have the property that although their dopingconcentration is approximately homogeneous in a radial direction, i.e.transversely with respect to a longitudinal direction x, the dopingconcentration varies greatly in the longitudinal direction x of thesemiconductor rod.

FIG. 2 schematically illustrates the resistivity and the dopingconcentration as a function of a position x of the rod in thelongitudinal direction. A length l of the rod 1 is for example 1200millimeters (mm), the resistivity at one end is for exampleapproximately 1470 ohm-cm, and the resistivity at the opposite end isfor example approximately 220 ohm-cm, that is to say is lower by morethan a factor of 6.

The resistivity is directly dependent on the doping concentration, theresistivity decreasing if the doping concentration increases. In theexample illustrated, it shall be assumed that the semiconductor rodconsists of silicon and that it has a basic doping of the n-typeproduced by n-doping phosphorus atoms as dopant atoms. For theabove-indicated values of the resistivity at the two ends, the dopantconcentration at said one end is for example approximately 3E12 cm⁻³,and at the opposite end for example approximately 2E13 cm⁻³. In a mannercorresponding to the electrical resistivity, therefore, the dopingconcentration over the entire length of the semiconductor rod alsovaries by more than a factor of 6. Furthermore, from semiconductor rodto semiconductor rod the maximum doping concentration (the minimumresistivity) and accordingly the minimum doping concentration (themaximum resistivity) also vary on account of conventional processfluctuations during the production of the doped semiconductor rods.

From the semiconductor rod illustrated schematically in FIG. 1,semiconductor wafers can be cut off or sawn off in a conventionalmanner. On account of the above-explained fluctuations of the dopingconcentration over the length of the semiconductor rod 1, the dopingconcentrations of the semiconductor wafers produced from such asemiconductor rod fluctuate to a considerable extent, such thatsemiconductor wafers produced from a semiconductor rod 1 that was dopedduring the production method, for the reasons explained in theintroduction, are primarily not suitable for the production ofsemiconductor components having a high dielectric strength.

Since semiconductor rods produced according to the MCZ method, and hencethe semiconductor wafers produced therefrom, can have a diameter of 12inches or more, with the use of such semiconductor wafers it is possibleto simultaneously produce a higher number of semiconductor componentsthan would be the case, for example, if smaller semiconductor rodsproduced according to the FZ (float-zone) method were used. Therefore,it is desirable to be able to use semiconductor rods produced accordingto the MCZ method (or generally semiconductor rods whose dopingfluctuates greatly in the longitudinal direction) for the production ofsemiconductor components having a high dielectric strength.

A description is given below of various methods which ultimately make itpossible to use such semiconductor wafers for the production ofsemiconductor components having a high dielectric strength. Thesemethods require firstly determining a doping concentration of the basicdoping of the individual semiconductor wafers. This process ofdetermining the doping concentration of the basic doping can be carriedout, for example, by measuring the resistivity of the individualsemiconductor wafers. Optical methods for determining the dopingconcentration, such as, for example, the surface photovoltage method orthe μ-PCD (Microwave Photoconductive Decay) method, are also possible.

The electrical resistivity can be measured for example by means of afour-tip measurement on the semiconductor wafer. FIG. 3 schematicallyshows a side view of such a semiconductor wafer 100 cut off from thesemiconductor rod 1. In a four-tip measurement, four electrodes thatusually have a contact tip are brought into contact with the surface ofthe semiconductor wafer 100. This bringing into contact is carried outat one of two main surfaces 101, 102 of the semiconductor wafer 100,which can also be designated as the front and rear sides of thesemiconductor wafer 100. FIG. 3 schematically illustrates a measuringarrangement 200 having four contact tips 201, 202, 203, 204. During themeasurement, via two of the contact tips, for example the contact tips201 and 204, a current is impressed into the semiconductor wafer 100 viathe surface, and a voltage is determined between the other two contacttips, such as the contact tips 202, 203, for example. On the basis ofthis measurement, it is possible to determine the electricalresistivity, in particular the electrical surface resistivity of thesemiconductor wafer 100. On the basis of the thickness of thesemiconductor wafer 100, that is to say the dimension of thesemiconductor wafer 100 in a direction perpendicular to the front andrear sides 101, 102, it is then possible to determine the electricalresistivity of the semiconductor wafer 100.

As already mentioned, the electrical resistivity is directly related tothe doping concentration of the basic doping, such that the dopingconcentration of the basic doping can be determined on the basis of theelectrical resistivity. For silicon having an n-type basic doping on thebasis of phosphorus atoms, the doping concentration of the basic dopingcan be determined depending on the electrical resistivity for examplewith the aid of the diagram in Sze: “Semiconductor Devices, Physics andTechnology”, 2^(nd) edition, 2002, Wiley-Verlag, ISBN 0-471-33372-7,page 55, FIG. 7.

As has already been mentioned above, the doping in the semiconductor rod1 in a radial direction is approximately homogeneous, such that thedoping concentration of the basic doping in the semiconductor wafer 100is approximately homogeneous, on condition that the thickness of thesemiconductor wafer 100 is small in comparison with the length l of thesemiconductor rod 1. Given a length l of the semiconductor rod 1 ofseveral 100 millimeters, such as, for example, 1000 millimeters or more,and a customary thickness of the semiconductor wafer 100 of less than 1mm, this condition is met. The above-explained measurement of thesurface resistivity, with the aim of determining the dopingconcentration of the basic doping, can be carried out at a plurality oflocations of the front side and/or rear side 101, 102 of thesemiconductor wafer 100, wherein the results thereby obtained for thesurface resistivity or—derived therefrom—the doping concentration of thebasic doping can be averaged. The result obtained by this averaging isthen the determined doping concentration of the basic doping of thesemiconductor wafer 100.

Instead of measuring the resistivity of the semiconductor wafer 100after the latter has been cut off from the semiconductor rod 1, there isalso the possibility of measuring the resistivity of a semiconductorwafer or of a plurality of semiconductor wafers even before they are cutoff from the semiconductor rod 1. Such a procedure is illustratedschematically in FIGS. 4A and 4B. FIG. 4A shows a side view of thesemiconductor rod 1 during the measurement, and FIG. 4B shows a planview of the semiconductor rod 1 during the measurement. In thismeasuring method, provision is made for subdividing the semiconductorrod 1 in the longitudinal direction x into a multiplicity ofsemiconductor sections (which are still fixedly connected and are partof the monocrystalline rod 1) and for determining at least once theelectrical resistivity, and thus the doping concentration of the basicdoping, within each of said sections. Afterward, the semiconductor rod 1is cut up in such a way that each of these individual sections forms asemiconductor wafer 100 ₁, 100 ₂, 100 ₃, 100 _(n), wherein theresistivity previously determined for a semiconductor section or thedoping concentration previously determined for said semiconductorsection is the determined resistivity or respectively the determineddoping concentration of the basic doping of the semiconductor waferobtained from said semiconductor section.

In the measuring method, provision is made for determining theresistivity of the semiconductor rod 1 at at least one position of thesemiconductor rod 1 in the longitudinal direction x, wherein theelectrical resistivity determined at this position is the electricalresistivity of the semiconductor wafer that is later cut off from thisposition of the semiconductor rod 1.

The measurement of the electrical resistivity of the semiconductor rod 1at one position or at a plurality of positions of the semiconductor rod1 in the longitudinal direction x thereof can be carried out by means ofa four-tip measurement, for example. Referring to FIG. 4B, themeasurement in this case takes place for example in such a way that thefour contact tips of the measuring device 200 act on the circumferentialsurface of the semiconductor rod 1 at a distance from one another in acircumferential direction of the semiconductor rod 1. In this method,the surface resistivity is measured, from which the electrical (volume)resistivity can be determined taking account of the geometry of thesemiconductor rod.

The method additionally provides for postdoping the semiconductor wafertaking account of the previously determined doping concentration of thebasic doping such that the semiconductor wafer 100 has a defined basicdoping, i.e. a defined doping concentration of the basic doping. Themethod provides, in particular, for performing an additional n-typedoping of the semiconductor wafer 100. In the case where an n-type basicdoping of the semiconductor wafer 100 is present, this additional n-typedoping can serve to obtain a higher n-type doping of the semiconductorwafer. In the case where a p-type basic doping of the semiconductorwafer 100 is present, this additional n-type doping can serve to reducea net p-type doping of the semiconductor wafer.

One possible method for postdoping the semiconductor wafer 100 isillustrated schematically in FIG. 5. This method provides for implantingprotons (hydrogen ions, H⁺) into the semiconductor wafer 100 via one ofthe front and rear sides 101, 102 and subsequently heating thesemiconductor wafer, such that the introduced protons in thesemiconductor lattice of the semiconductor crystal of the semiconductorwafer 100 form n-doping complexes, so-called hydrogen induced donors.The temperature of this thermal process is for example between 400° C.and 570° C., in particular between 450° C. and 550° C. The duration ofthe thermal process is for example between one hour and ten hours, inparticular between three hours and six hours. The additional doping ofthe semiconductor wafer 100 that is produced by this postdoping methodis an n-type doping, such that this method is suitable for increasingthe n-type doping of a semiconductor wafer 100 having a n-type basicdoping, or is suitable for reducing the p-type doping of a semiconductorwafer 100 having a p-type basic doping. In the case of a semiconductorwafer 100 having an n-type basic doping, the additional n-type dopingproduced by the proton irradiation and the thermal process is added tothe basic doping already present, such that the following holds trueafter the conclusion of the method explained with reference to FIG. 5:

N _(TOT) =N _(G) +N _(H)  (1),

wherein N_(TOT) is the total doping concentration, N_(G) denotes thedoping concentration of the basic doping, and N_(H) denotes the dopingconcentration added by the postdoping.

Since the basic doping N_(G) can differ greatly for individualsemiconductor wafers 100 in the manner explained, it is necessary forthe doping concentration N_(H) added by the postdoping to be adapted tothe basic doping N_(G) already present, in order to achieve a definedtotal doping of the semiconductor wafer 100.

In the method explained above with reference to FIG. 5, the dopingconcentration N_(H) of the added doping can be set by means of twoparameters, namely the implantation dose of the protons, that is to saythe quantity of protons implanted per unit area via the front sideand/or rear side 101, 102 and the temperature of the annealing process.It holds true here that, given a specific implantation dose, theresulting doping concentration is all the lower, the higher thetemperature of the annealing process is chosen within the temperaturerange indicated above. In one example of the method, provision is madefor choosing the implantation dose independently of the existing basicdoping N_(G) of the semiconductor wafer 100, i.e. for using the sameimplantation dose for each semiconductor wafer, and for adapting thetemperature of the annealing process depending on the previouslydetermined basic doping N_(G) in order thereby to set the added dopingconcentration N_(H).

The implantation method can be carried out in such a way that protonsare implanted into the semiconductor wafer 100 only with oneimplantation energy via one of the front and rear sides 101, 102 (thisside is subsequently designated as the implantation side). A maximum ofthe proton concentration then initially lies at a position in a verticaldirection of the semiconductor wafer 100 which is determined by theimplantation energy. This position is usually designated as theend-of-range or end-of-range region of the implantation. The “verticaldirection” of the semiconductor wafer 100 is a direction perpendicularto the front and rear sides 101, 102. During the subsequent thermalprocess, the protons then diffuse in the direction of the side via whichthe protons were implanted, and hydrogen induced donors arise fromcrystal defects produced by the proton irradiation in the crystallattice of the semiconductor wafer 100 and the protons. To what extentthe protons diffuse in the direction of the implantation side and howhomogeneous a doping is between the end-of-range and the implantationside depends, inter alia, on the duration of the thermal process and theposition of the end-of-range proceeding from the implantation side. Inprinciple, it holds true that the doping becomes more homogeneous as theduration of the thermal process increases, i.e. as the duration of theredistribution of the protons increases.

One example provides for carrying out the thermal process until thevolume between the implantation side and the end-of-range has an atleast approximately homogeneous doping. This should be understood tomean that at least 60% or even at least 80% of a volume of thesemiconductor wafer 100 between the implantation side and theend-of-range of the implantation has an at least approximatelyhomogeneous doping. In this connection, an “at least approximatelyhomogeneous doping” is a doping for which a ratio between a maximumdoping concentration and a minimum doping concentration in a volumeregion under consideration is less than 3, less than 2, less than 1.5 oreven less than 1.2.

A further method provides for carrying out a plurality of implantationsvia at least one of the front and rear sides 101, 102 and for varyingthe implantation energy in the process.

In a region between the end-of-range of the implantation and theopposite side of the semiconductor wafer 100 relative to theimplantation side, no crystal defects are produced by the protonimplantation, such that no hydrogen induced donors are formed there(despite a possible diffusion of the protons into this region). Oneexample of the method provides for implanting protons into thesemiconductor wafer 100 both via the front side 101 and via the rearside 102, wherein the implantation energies can be chosen in particularsuch that the end-of-range of the implantation via one of the front andrear sides lies closer to the other of the front and rear sides than theend-of-range of the implantation via said other of the front and rearsides. In this case, crystal defects are present in all regions of thesemiconductor wafer, such that an approximately homogeneous doping canbe achieved over the entire wafer 100.

Another example provides for carrying out implantation only via one sideand for removing (thinning) the semiconductor wafer 100, proceeding fromthe side via which implantation was not effected, as far as theend-of-range or even including the end-of-range. After the removal, theapproximately homogeneously doped region remains between the earlierend-of-range and the implantation side. The removal comprises, forexample, at least one of an etching method, a grinding method and apolishing method.

Two examples of a postdoping of the semiconductor wafer 100 using aproton implantation and a subsequent thermal process are explainedbelow. For explanation purposes, it shall be assumed that a total basicdoping (target doping concentration) of the semiconductor wafer of3.6E13 cm⁻³ is intended to be achieved over a depth of at least 120 μm.This basic doping corresponds to a resistivity of 120 ohm-cm. Theexamples explained below specify in each case the determined basicdoping of a semiconductor wafer 100 and the process parameters for thepostdoping (proton dose, implantation energy and duration andtemperature of the thermal process) which was carried out in order toachieve an at least approximately homogeneous doping with the targetdoping concentration in the semiconductor wafer 100 between theend-of-range of the implantation and the implantation side.

1^(St) Example

Determined basic doping: 3.08E13 cm⁻³(140 ohm-cm)Implantation energy: 4 MeVImplantation dose: 1E14 cm⁻²Duration of the thermal process: 10 hoursTemperature of the thermal process: 505° C.

2^(nd) Example

Determined basic doping: 2.16E13 cm⁻³(200 ohm-cm)Implantation energy: 4 MeVImplantation dose: 1.5E14 cm⁻²Duration of the thermal process: 8 hoursTemperature of the thermal process: 500° C.

The above-explained postdoping of the semiconductor wafer 100 can beperformed on the unprocessed semiconductor wafer 100, that is to saywhen the semiconductor wafer 100 has only the basic doping with whichthe semiconductor rod 1 was produced. In this case, however, there isthe risk that further process steps carried out for producing componentshaving a high dielectric strength will reduce the doping concentrationof the hydrogen induced donors in an undesired manner. In one exampleembodiment of the method, therefore, provision is made for performingthe postdoping of the semiconductor wafer 100 only when some processsteps for producing a semiconductor component having a high dielectricstrength have already been carried out. This is explained by way ofexample below with reference to FIGS. 6 to 10.

FIG. 6 schematically shows a plan view of a semiconductor wafer 100 onthe basis of which a multiplicity of semiconductor components having ahigh dielectric strength are produced. Dotted lines illustrate a gridthat defines a later subdivision of the semiconductor wafer 100 intoindividual semiconductor chips. Each of said semiconductor chips is thebasis of a semiconductor component having a high dielectric strength,such as, for example, a MOSFET, an IGBT, a diode or a thyristor. Thecomponent structures of the individual semiconductor chips are producedsimultaneously, wherein, for a given chip size, the yield ofsemiconductor chips per semiconductor wafer 100, and thus the efficiencyof the production method, increases as the diameter d of thesemiconductor wafer 100 increases.

FIGS. 7 to 10 illustrate as excerpts a vertical cross section of one ofthe semiconductor chips in a vertical sectional plane A-A illustratedschematically in FIG. 6. These figures illustrate an excerpt from aso-called inner region of the individual semiconductor chips, that is tosay a region in which active component regions of the semiconductorcomponent having a high dielectric strength that is implemented in thesemiconductor chip are arranged. So-called edge regions which surroundthe inner region in a ring-shaped manner and which have an edgetermination of the component are not illustrated in these figures.

FIG. 7 schematically shows a vertical cross-sectional view of a laterMOS transistor during the postdoping. Before the postdoping, i.e. beforethe proton implantation and the thermal process, in this case a drainregion 14 has already been produced in the region of one side 102 (whichis designated hereinafter as the rear side), of the wafer 100 and bodyregions 12, source regions 13 and gate electrodes 21 have already beenproduced in the region of a further side 101 (which is designatedhereinafter as the front side). The gate electrodes 21 are arrangedadjacent to the body region 12 and are dielectrically isolated fromsemiconductor regions in the wafer 100 by a gate dielectric 22. A sourceregion 13, a body region 12 and a gate electrode 21 are in each casepart of a so-called transistor cell, wherein the individual transistorcells have a drift region 11, which adjoins the body zones 12 of theindividual transistor cells, and the drain region 14 jointly. In thelater component, the individual transistor cells are connected inparallel by the gate electrodes 21 being jointly connected to a gateterminal and by the individual source regions being jointly connected toa source terminal. The body regions 12 can have contact regions 14 whichextend as far as the front side 101 and via which the body regions 12can likewise be connected to the source terminal.

The drain region 14 and the source and body regions 13, 12 can beproduced in a conventional manner by implantation and/or diffusionprocesses. The drift region 11 is a region having the basic doping ofthe semiconductor wafer 100, which before the postdoping is given onlyby the doping of the semiconductor rod 1. In the example illustrated,the gate electrodes 21 are trench electrodes, that is to say electrodesarranged in trenches of the semiconductor wafer 100. Such gateelectrodes 21 can be produced in a conventional manner by producingtrenches, producing a gate dielectric 22 on sidewalls and on the bottomof the trenches and by producing gate electrodes 21 on the gatedielectric layer 22. It goes without saying that other gate topologies,such as a planar gate electrode, for example, can also be provided.

During the postdoping, in the manner explained, protons are implantedinto the semiconductor wafer 100 via at least one of the front and rearsides 101, 102 and then the thermal process for producing the hydrogeninduced donors is carried out. In one example, the proton implantationis carried out via the front side 101 in such a way that the maximum ofthe proton concentration directly after the implantation, i.e. theend-of-range, is near the drain zone 14 or in the drain zone. In thiscase, during the thermal process the protons diffuse in the direction ofthe front side 101 and bring about an approximately homogeneouspostdoping of the drift zone 11. During this process, the body zone 12,the source zone 13 and the drain zone 14 can also be postdoped. However,the doping concentrations of these semiconductor zones are usuallygreater than the desired doping concentration of the postdoping by amultiple, such that the postdoping does not significantly influence thedoping concentrations of these semiconductor zones 12, 13, 14. In thisregard, the doping concentration of the drain zone 14 and of the sourcezone 13 lies above 10¹⁹ cm⁻³, for example, and the doping concentrationof the body zone 12 lies above 10¹⁶ cm⁻³, for example, while the desireddoping concentration of the postdoping lies in the range of between 10¹³cm⁻³ and 10¹⁴ cm⁻³, for example.

The MOS transistor component produced on the basis of the semiconductorwafer 100 in accordance with FIG. 7 can be a MOSFET. In this case, thesource zone 13 and the drain zone 14 are n-doped, while the body zone 12is p-doped. The component can also be embodied as an IGBT, wherein thedrain zone 14 is p-doped in this case. In the case of an IGBT, the drainzone 14 is also designated as the emitter zone. Moreover, in the case ofan IGBT, emitter short circuits can be present which extend from therear side 102 through the emitter zone 14 right into the drift zone 11and which are of the same conduction type as the drift zone 11 (thedrift zone 11 is also designated as the base zone in the case of anIGBT). Such emitter short circuits—like the emitter zone 14 as well—canalso be produced before the postdoping, but are not illustrated in FIG.7.

FIG. 7 additionally illustrates a drain metallization (emittermetallization) 31, which is applied to the rear side 102 of thesemiconductor body and makes contact with the drain region/emitterregion 14. Said metallization 31 can also be produced before thepostdoping is carried out, to be precise in particular if the protonimplantation is carried out via the front side 101 and if thesemiconductor wafer 100 is no longer thinned after the postdoping, asexplained below in connection with FIGS. 8A-8B.

A further example embodiment provides for implanting protons via therear side 102 or protons via the front and rear sides 101, 102. In thiscase, the metallization 31 is produced after the postdoping has beencarried out. The same correspondingly applies to metallizations in theregion of the front side 101 which form the later gate terminal and thelater source terminal of the component.

FIGS. 8A and 8B illustrate a further method for a postdoping of asemiconductor wafer 100 during a production of a MOS transistor. Thesefigures in each case show a vertical cross section of an excerpt fromthe semiconductor wafer 100 during the method.

This method provides for thinning the semiconductor wafer after thepostdoping proceeding from the rear side 102. FIG. 8A shows thesemiconductor wafer 100 during the proton implantation of thepostdoping, wherein, for example, the source and body regions 13, 12 andthe gate electrode 21 and the gate dielectric 22 have already beenproduced beforehand in the region of the front side 101. In the exampleillustrated, the proton implantation is carried out via the front side101, and the end-of-range region of the implantation is designated by110 in FIG. 8A. Between said end-of-range region 110 of the implantationand the rear side 102, hardly any hydrogen induced donors are formedduring the thermal process, i.e. the doping is not adapted in thisregion.

This region between the rear side 102 and the end-of-range region 110 orbetween the rear side 102 and including the end-of-range region issubsequently removed by the removal of the semiconductor wafer 100proceeding from the rear side 102. The reference sign 102′ in FIG. 8Bdesignates the rear side obtained after this removal. Dopant atoms forproducing the drain or emitter zone 14 are subsequently introduced viathis rear side 102′. These dopants are implanted, for example. Thesedopants can be activated by a laser beam or by an RTA (Rapid ThermalAnnealing) process. In this case, only a near-surface region of thesemiconductor wafer 100 at the rear side 102′ is heated briefly, and sothis activation has no appreciable influence on the postdoping withhydrogen induced donors. “Near-surface” regions are, for example, thosewhich are nearer than 1 micrometer or nearer than 0.5 micrometer to thesurface. The thermal process for producing the hydrogen induced donorscan be carried out before or after the activation.

As an alternative to the method explained with reference to FIGS. 7 and8A-8B, the proton implantation for the postdoping can also be carriedout via the rear side 102, for example before the rear-sidemetallization 31 is applied. In this case, the end-of-range region ofthe implantation lies in the body regions 12, for example. The drain oremitter region 14 can be produced before or after the protonimplantation, wherein, in the last-mentioned case, the semiconductorwafer 100 can also be thinned proceeding from the rear side 102 beforethe production of the drain or emitter region 14.

FIG. 9 schematically shows a vertical cross-sectional view of a portionof the semiconductor wafer 100 in which active component regions of athyristor were produced before the postdoping. The active componentregions are a p-type emitter 42 in the region of the rear side 102 ofthe semiconductor wafer 100, a p-type base 41 in the region of the frontside 101, and n-type emitters 43 arranged in the p-type base 41. Thep-type emitter 42, the p-type base 41 and the n-type emitters 43 can beproduced in a conventional manner by implantation and/or diffusionprocesses. Arranged between the p-type emitter 42 and the p-type base 41there is an n-type base 11, which has a doping which corresponds to thebasic doping of the semiconductor wafer 100 and the doping of which isadapted by the postdoping. The doping concentrations of the p-type base42, of the n-type emitters 43 and of the p-type emitter 42 can also bealtered by the postdoping, but the doping concentrations of thesecomponent regions are significantly higher than the doping concentrationprovided by the postdoping, and so no significant change in the dopingconcentrations of these semiconductor regions 41-43 occurs.

As also in the case of the example embodiment explained above withreference to FIG. 7, the protons can be implanted into the semiconductorwafer 100 via the front side 101, the rear side 102 or via the front andrear sides 101, 102. If protons are implanted only via one of the frontand rear sides 101, 102 and if no thinning proceeding from the rear sideis carried out, a metallization can already been applied to the other ofsaid front and rear sides 101, 102 before the postdoping is carried out.The p-type emitter 43 can—in a manner corresponding to the drain oremitter region in accordance with FIGS. 7 and 8A-8B—be produced beforeor after the postdoping.

The doping concentration of the p-type emitter 42 and of the n-typeemitters 43 is for example in the range of the doping concentration ofthe source and drain regions 13, 14 of the component in accordance withFIG. 7, and the doping concentration of the p-type base 41 is forexample in the range of the doping concentration of the body zone 12 inaccordance with FIG. 7.

FIG. 10 shows a component structure of a bipolar diode during thepostdoping. This bipolar diode comprises an n-type base 11, the dopingof which corresponds to the basic doping of the semiconductor wafer 100,and a first emitter 51, for example an n-type emitter 51, in the regionof the front side 101 of the semiconductor wafer and a second emitter52, such as, for example, a p-type emitter, in the region of the rearside 102 of the semiconductor wafer 100. These two emitters 51, 52 canbe produced in a conventional manner by implantation and/or diffusionprocesses. By means of the postdoping with the proton implantation andthe thermal process, the doping concentration of the n-type base 11 ispostdoped. In this case, the two emitters 51, 52 can also be postdoped.However, the doping concentration thereof is significantly higher thanthe doping concentration produced by the postdoping, and so nosignificant change in the doping concentrations of the two emitters 51,52 occurs as a result of the postdoping. In this regard, the dopingconcentration of the two emitters 51, 52 is more than 10¹⁹ cm⁻³, forexample, while the doping concentration of the postdoping is onlybetween 10¹³ cm⁻³ and 10¹⁴ cm⁻³, for example. The proton implantation,as also in the case of the methods explained above with reference toFIGS. 7 and 8, can be carried out only via one of the front and rearsides 101, 102 or via both of said front and rear sides 101, 102. If aproton implantation is carried out only via one of these two sides, ametallization 31 can already be produced on the other of these twosides, such as the rear side 102, for example, before the postdoping iscarried out, provided that the semiconductor wafer 100 is no longerthinned. The rear-side emitter 52 can—in a manner corresponding to thedrain or emitter region in accordance with FIGS. 7 and 8A-8B—be producedbefore or after the postdoping.

FIG. 11 shows a vertical cross-sectional view of a later Schottky diodeduring the postdoping. In this case, the semiconductor wafer 100 has ann-type emitter 61 in the region of the rear side 102 which adjoins then-type base 11. The n-type base 11 has a doping which corresponds to thebasic doping of the semiconductor wafer 100 and the doping of which isadapted by the postdoping. In later method steps, a Schottky metal isthen applied on the front side 101 of the semiconductor wafer 100. Therear-side emitter 61 can—in a manner corresponding to the drain oremitter region in accordance with FIGS. 7 and 8A-8B—be produced beforeor after the postdoping.

In the method explained, the total doping concentration of thesemiconductor wafer is composed of the original doping concentrationresulting from the process for producing the semiconductor wafer 100 orthe semiconductor rod 1 and the doping concentration added by thepostdoping. One example of the method provides for the original basicdoping concentration already to make up at least 20%, at least 40% or atleast 60% of the total doping concentration. Correspondingly, thepostdoping contributes a maximum of 80%, a maximum of 60% or a maximumof 40% to the total doping concentration after the postdoping has beencarried out. The presence of a basic doping of the order of magnitudementioned above has the result that parasitic effects, such as, forexample, the presence of oxygen in the semiconductor wafer, have a lessdisturbing effect than in a comparative case in which, proceeding froman intrinsic semiconductor wafer, the doping is brought about only byhydrogen induced donors. Moreover, the costs of the postdoping are lowerthan in a method in which, proceeding from an intrinsic semiconductorwafer, the doping is brought about only by hydrogen induced donors,since the required proton implantation dose is significantly lower thanfor the case of a pure proton doping.

As an alternative or in addition to a postdoping comprising a protonimplantation and a thermal process, a postdoping of the semiconductorwafer 100 can also be effected by means of a neutron irradiation. Duringthe irradiation of a semiconductor wafer 100 consisting of silicon withneutrons, radioactive silicon-31(31Si) arises, which decays to n-dopingphosphorus with a half-life of approximately 2.6 hours and with emissionof beta radiation. In this method, the doping concentration of thepostdoping can be set by means of the irradiation dose of the neutrons.For the total doping after carrying out such a postdoping method using aneutron irradiation, the following holds true:

N _(TOT) =N _(G) +N _(N)  (2),

wherein N_(TOT) is the total basic doping after the postdoping has beencarried out, N_(G) is the basic doping before the postdoping and N_(N)is the postdoping brought about by the neutron implantation.

Referring to FIG. 12, which schematically shows a vertical cross sectionthrough the semiconductor wafer 100, the neutrons can be implanted intothe semiconductor wafer 100 via the front side 101 and/or the rear side102. The neutron irradiation takes place in a nuclear reactor, forexample.

Since neutrons penetrate far into the semiconductor wafer even with acomparatively low implantation energy, there is also the possibility ofcarrying out the postdoping for a plurality of semiconductor waferssimultaneously, which jointly form a monocrystalline portion of theoriginal semiconductor rod 1. FIG. 13 schematically shows a verticalcross section through such a portion of the original semiconductor rod1. This portion of the semiconductor rod 1 comprises a plurality ofsubportions that respectively form a later semiconductor wafer 100 ₁,100 ₂, 100 ₃. These are three semiconductor wafers in the case of theexemplary embodiment illustrated in FIG. 13. However, this is only anexample; provision can also be made of more than three subportions thatrespectively later form a semiconductor wafer. The neutron irradiationcan be carried out via a front side 101′ and/or a rear side 102′ of therod portion 100′. Alternatively or supplementarily, the neutronirradiation can also be carried out via the side wall for the case ofthe doping of a rod portion.

Since a thermally stable n-type doping results from the neutronimplantation, the postdoping using the neutron implantation can alreadybe carried out on the end process semiconductor wafer 100, i.e. beforeimplantation and/or diffusion processes for producing active componentregions are actually carried out. Optionally, a specific heat treatmentstep for activating the phosphorus doping and for annealing theradiation damage can be carried out, wherein the temperatures can bebetween 800° C. and 1000° C. and the duration is one or more hours, forexample. However, it is also possible to use for this purposetemperature steps which are carried out during the production of asemiconductor component for other purposes, such as, for example, foractivating or indiffusing implanted dopants.

The method explained above makes it possible to individually adapt thedoping concentration of a semiconductor wafer which already has a basicdoping.

In the method, there is the possibility, in particular, of settingdifferent total dopings for different semiconductor wafers from asemiconductor rod, depending on the requirement. In this regard, by wayof example, semiconductor wafers having a low basic doping can bepostdoped such that they have a first total doping concentration, whilesemiconductor wafers which already have a higher basic doping can bepostdoped such that they have a second total doping concentration,higher than the first total doping concentration. The individual wafersfrom a rod can be grouped, for example, wherein the dopings of thewafers of the different groups are adapted to different total dopingconcentrations, for example.

1. A method for treating a semiconductor wafer having a basic doping,wherein the method comprises: determining a doping concentration of thebasic doping; and adapting the basic doping of the semiconductor wafer(100) by postdoping comprising at least one of the following methods: aproton implantation and a subsequent thermal process for producinghydrogen induced donors, and a neutron irradiation; wherein at least oneof the following parameters is dependent on the determined dopingconcentration of the basic doping: an implantation dose of the protonimplantation, a temperature of the thermal process, and an irradiationdose of the neutron irradiation.
 2. The method as claimed in claim 1,wherein the temperature during the thermal process is between 400° C.and 570° C. or between 450° C. and 550° C.
 3. The method as claimed inclaim 2, wherein the duration of the thermal process is between one hourand ten hours or between three hours and six hours.
 4. The method asclaimed in claim 1, wherein after the neutron irradiation anotherthermal process is carried out, wherein the semiconductor wafer isheated to a temperature of between 800° C. and 1000° C.
 5. The method asclaimed in claim 4, wherein a duration of the another thermal process isbetween one hour and ten hours.
 6. The method as claimed in claim 1,wherein the semiconductor wafer has a first side, and wherein the protonimplantation is carried out via the first side.
 7. The method as claimedin claim 6, wherein the proton implantation comprises at least twoproton implantation acts in which protons are implanted with differentimplantation energies.
 8. The method as claimed in claim 1, wherein thesemiconductor wafer has a first side, and wherein the neutronirradiation is carried out via the first side.
 9. The method as claimedin claim 1, wherein the semiconductor wafer is part of a single crystalwhich comprises at least two semiconductor wafers and which has a firstside, and wherein the neutron irradiation is carried out via the firstside of the single crystal.
 10. The method as claimed in claim 1,wherein determining the doping concentration of the basic doping of thesemiconductor wafer comprises a measurement of a resistivity of thesemiconductor wafer.
 11. The method as claimed in claim 10, wherein thesemiconductor wafer is a semiconductor wafer obtained by dividing acylindrical single crystal, and wherein the resistivity is measuredafter the single crystal has been divided.
 12. The method as claimed inclaim 10, wherein the semiconductor wafer is a semiconductor waferobtained by dividing a cylindrical single crystal, and wherein theresistivity is measured before the single crystal is divided.
 13. Themethod as claimed in claim 1, wherein the basic doping is an n-typebasic doping.
 14. The method as claimed in claim 13, wherein the basicdoping is formed by phosphorus atoms.
 15. The method as claimed in claim1, wherein the basic doping is a p-type basic doping.
 16. The method asclaimed in claim 11, wherein the doping concentration of the basicdoping is higher than 1E13 cm⁻³.
 17. The method as claimed in claim 16,wherein the doping concentration of the basic doping is higher than 1E12cm⁻³.
 18. The method as claimed in claim 1, wherein a dopingconcentration of the basic doping before the adaptation is between 20%to 60% of a doping concentration after the adaptation.
 19. The method asclaimed in claim 1, wherein protons are implanted during the protonimplantation via a first side of the semiconductor wafer into anend-of-range region of the semiconductor wafer, and wherein the thermalprocess is chosen such that a doping concentration added by theadaptation is approximately homogeneous in at least between 60% to atleast 80% of a volume of the semiconductor wafer in a region between theend-of-range region and the first side.
 20. The method as claimed inclaim 17, wherein a ratio between a maximum doping concentration and aminimum doping concentration in the at least approximately homogeneouslydoped volume is less than 1.2.
 21. The method as claimed in claim 1,wherein protons are implanted during the proton implantation via a firstside of the semiconductor wafer into an end-of-range region, and whereinthe semiconductor wafer is removed at least as far as the end-of-rangeregion proceeding from a second side situated opposite the first side.22. The method as claimed in claim 1, wherein the semiconductor wafer isa semiconductor wafer produced according to the magnetic Czochralski(MCZ) method.
 23. The method as claimed in claim 1, wherein thesemiconductor wafer has a diameter of 12 inches or more.